The present invention relates to an improved method of electrolytic deposition of a conductive polymer onto a non-conductive substrate. More specifically, the present invention relates to an aqueous solution comprising monomer and surfactants whereby a conductive polymer can be deposited on a non-conductive substrate efficiently without the necessity for a continuous seed layer.
Solid state electrolytic capacitors are well known in the art. The capacitors typically comprise an anode, or positive electrode, made from a porous valve-action metal. A solid electrolyte forms a cathode, or negative electrode. An oxide film, typically formed as the oxide of the anode, acts as a dielectric between the cathode and anode. Other components are typically included such as leads, encasement layers and the like.
Manganese dioxide has been employed in the past as a solid state conductor in capacitors for many years. The tendency of manganese dioxide to support ignition upon failure of the capacitor has led investigators to search for less powerful oxidizing agents which can be used as solid state conductors. This research has led to the widespread use of conductive polymers such as polypyrroles, polyanilines and polythiophenes.
Electrodeposition of a monomer onto an oxide surface, and electrolytic polymerization thereof has been difficult due, in part, to the inherent inability of an oxide layer to conduct. To overcome these problems it has been standard practice in the art to form a continuous seed layer of a suitable conductor, such as manganese dioxide, on the non-conductive oxide layer prior to electrodeposition of the intrinsically conductive polymer. A bothersome requirement for this process is the necessity for a continuous deposit of the manganese dioxide layer. Where voids exist in the manganese dioxide layer the electrodeposition is insufficient thereby resulting in poor coverage of polymer and failed capacitors. The desire to decrease manufacturing losses therefore led those skilled in the art to err on the side of a thicker manganese dioxide coating to avoid discontinuities in the seed layer. This increases both the amount of material required and the manufacturing complexity. The thicker manganese dioxide is a manufacturing necessity which does not enhance the product as viewed by the end user. It is therefore a desire to decrease the amount of the manganese dioxide seed layer without compromising the manufacturing yields or product performance. These desires have been contradictory prior to the present invention.
The present invention circumvents the problems in the art by providing a technique for electrodeposition whereby the conductive polymer can be adequately deposited on the surface of the dielectric and the electrodeposited conducted polymer can be deposited on a discontinuous seed layer. This advance greatly increases manufacturing efficiency by allowing lower seed layer loading on the surface of the dielectric.